The present invention relates to a circularly polarizing plate comprising a linearly polarizing membrane and a quarter wave plate.
A circularly polarizing plate has a function of extracting circularly polarized light from non-polarized light. The circularly polarizing plate usually comprises a linearly polarizing membrane and a quarter wave (xcex/4) plate. The linearly polarizing membrane has a function of extracting linearly polarized light from non-polarized light, and the quarter wave plate has a function of converting the extracted linearly polarized light to circularly polarized light.
The circularly polarizing plate has been used in various optical devices such as a reflective liquid crystal display, a pick up device for recording an optical disc, an organic electroluminescence (EL) display or an antireflection film. The optical characteristic of the circularly polarizing plate depends on the function of the quarter wave plate.
Various quarter wave plates have been proposed and used in optical devices. However, most of the conventional plates can function as a quarter wave plate merely at a specific wavelength. If a quarter wave plate functions merely at a specific wavelength, a circularly polarizing plate using the quarter wave plate should function merely at the specific wavelength.
Japanese Patent Provisional Publication No. 5(1993)-27118 discloses a quarter wave plate comprising two polymer films, in which an angle between the slow axes of the films is essentially 90xc2x0. The disclosed quarter wave plate can function within a wide wavelength range.
Each of Japanese Patent Provisional Publication Nos. 10(1998)-68816 and 10(1998)-90521 discloses a quarter wave plate comprising two polymer films, in which an angle between the slow axes of the films is essentially 60xc2x0. The disclosed quarter wave plate can also function within a wide wavelength range.
Each of the quarter wave plates disclosed in Japanese Patent Provisional Publication Nos. 5(1993)-27118, 10(1998)-68816 and 10(1998)-90521 can function within a wide wavelength range. However, the quarter wave plate comprises two polymer films. The directions of the slow axes of two polymer films should be adjusted to obtain a quarter wave plate. It is difficult to adjust the directions of two polymer films. A circularly polarizing plate further comprises a linearly polarizing membrane, which is also made of a polymer film (usually a polyvinyl alcohol film). The direction of the transparent axis of the linearly polarizing membrane should also be adjusted to obtain a circularly polarizing plate. It is more difficult to adjust the directions of three polymer films.
In preparation of the circularly polarizing plate comprising three polymer films, the films should be cut into chips (small pieces) along a predetermined direction, and then pasted together while adjusting the directions of the slow axes and the transparent axis. The process of pasting the chips together is troublesome. The optical characteristic of the plate is sometimes degraded where the axes are shifted in the process. The troublesome process decreases the yield of the plate, and increases the cost. Further, the chips are sometimes contaminated at the troublesome process.
Further, the circularly polarizing plate is rather thick because the plate comprises three polymer films.
An object of the present invention is to provide a circularly polarizing plate which functions within a wide wavelength range.
Another object of the invention is to provide a circularly polarizing plate which can easily be prepared without a troublesome process.
A further object of the invention is to provide a thin circularly polarizing plate.
The present invention provides a circularly polarizing plate which comprises a linearly polarizing membrane and a quarter wave plate comprising an optically anisotropic layer A and an optically anisotropic layer B, said quarter wave plate having such an optical characteristic that a retardation value essentially is a quarter of a wavelength when the retardation value is measured at the wavelength of 450 nm, 550 nm and 650 nm, wherein one of the optically anisotropic layers A and B is a layer made from liquid crystal molecules, and the other is a polymer film or a layer made from liquid crystal molecules.
The circularly polarizing plate of the present invention can function within a wide wavelength range (450 nm to 650 nm). The plate of the invention is characterized in that at least one of the optically anisotropic layers A and B is made from liquid crystal molecules.
The applicants have found that liquid crystal molecules are advantageously used in an optically anisotropic layer of the circularly polarizing plate. The direction of the slow axis of a layer made from liquid crystal molecules can easily be adjusted by controlling the alignment of the liquid crystal molecules. Therefore, the circularly polarizing plate of the present invention can easily be prepared without a troublesome process such as a process of cutting films into chips or a process of pasting chips together.
Further, liquid crystal molecules have a high optical anisotropy, compared with a polymer film. Accordingly, an optically anisotropic layer made from the liquid crystal molecules is usually thinner than a layer made of a polymer film. Therefore, a thin circularly polarizing plate can be obtained according to the present invention.